Water of crystallization

Template:Short description In chemistry, water(s) of crystallization or water(s) of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions.<ref>Template:Greenwood&Earnshaw2nd</ref> In some contexts, water of crystallization is the total mass of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation.

Upon crystallization from water, or water-containing solvents, many compounds incorporate water molecules in their crystalline frameworks. Water of crystallization can generally be removed by heating a sample but the crystalline properties are often lost.

Compared to inorganic salts, proteins crystallize with large amounts of water in the crystal lattice. A water content of 50% is not uncommon for proteins.

Applications

Knowledge of hydration is essential for calculating the masses for many compounds. The reactivity of many salt-like solids is sensitive to the presence of water. The hydration and dehydration of salts is central to the use of phase-change materials for energy storage.<ref>Template:Cite journal</ref>

Position in the crystal structure

File:H-bondingFeSO47aq.tif

Some hydrogen-bonding contacts in Template:Chem2. This metal aquo complex crystallizes with water of hydration, which interacts with the sulfate and with the Template:Chem2 centers.

A salt with associated water of crystallization is known as a hydrate. The structure of hydrates can be quite elaborate, because of the existence of hydrogen bonds that define polymeric structures.<ref>Template:Cite journal</ref> <ref>Template:Cite journal</ref> Historically, the structures of many hydrates were unknown, and the dot in the formula of a hydrate was employed to specify the composition without indicating how the water is bound. Per IUPAC's recommendations, the middle dot is not surrounded by spaces when indicating a chemical adduct.<ref>Template:Cite book</ref> Examples:

• Template:Chem2 – copper(II) sulfate pentahydrate
• Template:Chem2 – cobalt(II) chloride hexahydrate
• Template:Chem2 – tin(II) (or stannous) chloride dihydrate

For many salts, the exact bonding of the water is unimportant because the water molecules are made labile upon dissolution. For example, an aqueous solution prepared from Template:Chem2 and anhydrous Template:Chem2 behave identically. Therefore, knowledge of the degree of hydration is important only for determining the equivalent weight: one mole of Template:Chem2 weighs more than one mole of Template:Chem2. In some cases, the degree of hydration can be critical to the resulting chemical properties. For example, anhydrous Template:Chem2 is not soluble in water and is relatively useless in organometallic chemistry whereas Template:Chem2 is versatile. Similarly, hydrated Template:Chem2 is a poor Lewis acid and thus inactive as a catalyst for Friedel-Crafts reactions. Samples of Template:Chem2 must therefore be protected from atmospheric moisture to preclude the formation of hydrates.

File:Ca(aq)6 improved image.tif

Structure of the polymeric Template:Chem2 center in crystalline calcium chloride hexahydrate. Three water ligands are terminal, three bridge. Two aspects of metal aquo complexes are illustrated: the high coordination number typical for Template:Chem2 and the role of water as a bridging ligand.

Crystals of hydrated copper(II) sulfate consist of Template:Chem2 centers linked to Template:Chem2 ions. Copper is surrounded by six oxygen atoms, provided by two different sulfate groups and four molecules of water. A fifth water resides elsewhere in the framework but does not bind directly to copper.<ref>Template:Cite book</ref> The cobalt chloride mentioned above occurs as Template:Chem2 and Template:Chem2. In tin chloride, each Sn(II) center is pyramidal (mean Template:Chem2 angle is 83°) being bound to two chloride ions and one water. The second water in the formula unit is hydrogen-bonded to the chloride and to the coordinated water molecule. Water of crystallization is stabilized by electrostatic attractions, consequently hydrates are common for salts that contain +2 and +3 cations as well as −2 anions. In some cases, the majority of the weight of a compound arises from water. Glauber's salt, Template:Chem2, is a white crystalline solid with greater than 50% water by weight.

Consider the case of nickel(II) chloride hexahydrate. This species has the formula Template:Chem2. Crystallographic analysis reveals that the solid consists of Template:Nowrap subunits that are hydrogen bonded to each other as well as two additional molecules of Template:Chem2. Thus one third of the water molecules in the crystal are not directly bonded to Template:Chem2, and these might be termed "water of crystallization".

Analysis

The water content of most compounds can be determined with a knowledge of its formula. An unknown sample can be determined through thermogravimetric analysis (TGA) where the sample is heated strongly, and the accurate weight of a sample is plotted against the temperature. The amount of water driven off is then divided by the molar mass of water to obtain the number of molecules of water bound to the salt.

Other solvents of crystallization

Water is particularly common solvent to be found in crystals because it is small and polar. But all solvents can be found in some host crystals. Water is noteworthy because it is reactive, whereas other solvents such as benzene are considered to be chemically innocuous. Occasionally more than one solvent is found in a crystal, and often the stoichiometry is variable, reflected in the crystallographic concept of "partial occupancy". It is common and conventional for a chemist to "dry" a sample with a combination of vacuum and heat "to constant weight".

For other solvents of crystallization, analysis is conveniently accomplished by dissolving the sample in a deuterated solvent and analyzing the sample for solvent signals by NMR spectroscopy. Single crystal X-ray crystallography is often able to detect the presence of these solvents of crystallization as well. Other methods may be currently available.

Table of crystallization water in some inorganic halides

In the table below are indicated the number of molecules of water per metal in various salts.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Hydrated metal halides and their formulas	Coordination sphere of the metal	Equivalents of water of crystallization that are not bound to M	Remarks
Calcium chloride Template:Chem2	Template:Chem2	Template:CNone	example of water as a bridging ligand<ref>Template:Cite journal</ref>
Titanium(III) chloride Template:Chem2	trans-Template:Chem2<ref name=VX3aq6/>	two	isomorphous with Template:Chem2
Titanium(III) chloride Template:Chem2	Template:Chem23+<ref name=VX3aq6/>	Template:CNone	isomeric with Template:Chem2.2H2O<ref>Template:Greenwood&Earnshaw2nd</ref>
Zirconium(IV) fluoride Template:Chem2	Template:Chem2	Template:CNone	page=965">Template:Greenwood&Earnshaw2nd</ref>
Hafnium tetrafluoride Template:Chem2	Template:Nowrap	one	page=965"/>
Vanadium(III) chloride Template:Chem2	trans-Template:Chem2<ref name=VX3aq6/>	two
Vanadium(III) bromide Template:Chem2	trans-Template:Chem2<ref name=VX3aq6>Template:Cite journal</ref>	two
Vanadium(III) iodide Template:Chem2	Template:Chem2	Template:CNone	relative to Template:Chem2 and Template:Chem2, Template:Chem2 competes poorly with water as a ligand for V(III)
Template:Chem2	Template:Chem2	four
Chromium(III) chloride Template:Chem2	trans-Template:Chem2	two	dark green isomer, aka "Bjerrums's salt"
Chromium(III) chloride Template:Chem2	Template:Chem2	one	blue-green isomer
Chromium(II) chloride Template:Chem2	trans-Template:Chem2	Template:CNone	square planar/tetragonal distortion
Chromium(III) chloride Template:Chem2	Template:Chem2	Template:CNone	violet isomer. isostructural with aluminium compound<ref>Template:Cite journal</ref>
Manganese(II) chloride Template:Chem2	trans-Template:Chem2	two
Manganese(II) chloride Template:Chem2	cis-Template:Chem2	Template:CNone	cis molecular, the unstable trans isomer has also been detected<ref>Template:Cite journal</ref>
Manganese(II) bromide Template:Chem2	cis-Template:Chem2	Template:CNone	cis, molecular
Manganese(II) iodide Template:Chem2	trans-Template:Chem2	Template:CNone	molecular, isostructural with FeCl2(H2O)4.<ref>Template:Cite journal</ref>
Manganese(II) chloride Template:Chem2	trans-Template:Chem2	Template:CNone	polymeric with bridging chloride
Manganese(II) bromide Template:Chem2	trans-Template:Chem2	Template:CNone	polymeric with bridging bromide
Rhenium(III) chloride Template:Chem2	triangulo-Template:Chem2	Template:CNone	heavy early metals form M-M bonds<ref>Template:Cite journal</ref>
Iron(II) chloride Template:Chem2	trans-Template:Chem2	two
Iron(II) chloride Template:Chem2	trans-Template:Chem2	Template:CNone	molecular
Iron(II) bromide Template:Chem2	trans-Template:Chem2	Template:CNone	molecular,<ref name=Kenji/> hydrates of FeI2 are not known
Iron(II) chloride Template:Chem2	trans-Template:Chem2	Template:CNone	polymeric with bridging chloride
Iron(III) chloride Template:Chem2	trans-Template:Chem2	two	one of four hydrates of ferric chloride,<ref name=Cotton>Template:Cite journal</ref> isostructural with Cr analogue
Iron(III) chloride Template:Chem2	cis-Template:Chem2	two	the dihydrate has a similar structure, both contain Template:Chem2 anions.<ref name=Cotton/>
Cobalt(II) chloride Template:Chem2	trans-Template:Chem2	two
Cobalt(II) bromide Template:Chem2	trans-Template:Chem2	two
Cobalt(II) iodide Template:Chem2	Template:Chem2	Template:CNone<ref name=Lou>Template:Cite journal</ref>	iodide competes poorly with water
Cobalt(II) bromide Template:Chem2	trans-Template:Chem2	Template:CNone	molecular<ref name=Kenji/>
Cobalt(II) chloride Template:Chem2	cis-Template:Chem2	Template:CNone	note: cis molecular
Cobalt(II) chloride Template:Chem2	trans-Template:Chem2	Template:CNone	polymeric with bridging chloride
Cobalt(II) bromide Template:Chem2	trans-Template:Chem2	Template:CNone	polymeric with bridging bromide
Nickel(II) chloride Template:Chem2	trans-Template:Chem2	two
Nickel(II) chloride Template:Chem2	cis-Template:Chem2	Template:CNone	note: cis molecular<ref name=Kenji>Template:Cite journal</ref>
Nickel(II) bromide Template:Chem2	trans-Template:Chem2	two
Nickel(II) iodide Template:Chem2	Template:Chem2	Template:CNone<ref name=Lou/>	iodide competes poorly with water
Nickel(II) chloride Template:Chem2	trans-Template:Chem2	Template:CNone	polymeric with bridging chloride
Platinum(IV) chloride Template:Chem2<ref>Template:Cite journal</ref>	trans-Template:Chem2	3	octahedral Pt centers; rare example of non-first row chloride-aquo complex
Platinum(IV) chloride Template:Chem2<ref>Template:Cite journal</ref>	fac-Template:Chem2	0.5	octahedral Pt centers; rare example of non-first row chloride-aquo complex
Copper(II) chloride Template:Chem2	Template:Chem2	Template:CNone	tetragonally distorted two long Cu-Cl distances
Copper(II) bromide Template:Chem2	Template:Chem2	two	tetragonally distorted two long Cu-Br distances<ref name=Kenji/>
Zinc(II) chloride ZnCl2(H2O)1.33<ref name=Follner>Template:Cite journal</ref>	Template:Chem2	Template:CNone	coordination polymer with both tetrahedral and octahedral Zn centers
Zinc(II) chloride ZnCl2(H2O)2.5<ref>Template:Cite journal</ref>	Template:Chem2	Template:CNone	tetrahedral and octahedral Zn centers
Zinc(II) chloride Template:Chem2<ref name=Follner/>	Template:Chem2	Template:CNone	tetrahedral and octahedral Zn centers
Zinc(II) chloride ZnCl2(H2O)4.5<ref name=Follner/>	Template:Chem2	three	tetrahedral and octahedral Zn centers
Cadmium chloride CdCl2·H2O<ref name="mono">Template:Cite journal</ref>		Template:CNone	water of crystallization is rare for heavy metal halides
Cadmium chloride CdCl2·2.5H2O<ref>Template:Cite journal</ref>	CdCl5(H2O) & CdCl4(H2O)2	Template:CNone
Cadmium chloride CdCl2·4H2O<ref name="tet">Template:Cite journal</ref>		Template:CNone	octahedral
Cadmium bromide CdBr2(H2O)4<ref>Template:Cite journal</ref>	Template:Chem2	two	octahedral Cd centers
Aluminum trichloride Template:Chem2	Template:Chem2	Template:CNone	isostructural with the Cr(III) compound

Examples are rare for second and third row metals. No entries exist for Mo, W, Tc, Ru, Os, Rh, Ir, Pd, Hg, Au. AuCl₃(H₂O) has been invoked but its crystal structure has not been reported.

Hydrates of metal sulfates

Transition metal sulfates form a variety of hydrates, each of which crystallizes in only one form. The sulfate group often binds to the metal, especially for those salts with fewer than six aquo ligands. The heptahydrates, which are often the most common salts, crystallize as monoclinic and the less common orthorhombic forms. In the heptahydrates, one water is in the lattice and the other six are coordinated to the ferrous center.<ref>Template:Cite journal</ref> Many of the metal sulfates occur in nature, being the result of weathering of mineral sulfides.<ref name=asia>Template:Cite journal</ref><ref name=Redhammer/> Many monohydrates are known.<ref name=Kieserite/>

Formula of hydrated metal ion sulfate	Coordination sphere of the metal ion	Equivalents of water of crystallization that are not bound to M	mineral name	Remarks
MgSO4(H2O)	Template:Nowrap<ref name=Kieserite>Template:Cite journal</ref>	Template:CNone	kieserite	see Mn, Fe, Co, Ni, Zn analogues File:ICSD CollCode71346.png Substructure of MSO4(H2O), illustrating presence of bridging water and bridging sulfate (M = Mg, Mn, Fe, Co, Ni, Zn).
MgSO4(H2O)4	[Mg(H2O)4(κ′,κ1-SO4)]2	Template:CNone		sulfate is bridging ligand, 8-membered Mg2O4S2 rings<ref name="ReferenceA">Template:Cite journal</ref>
MgSO4(H2O)6	[Mg(H2O)6]	Template:CNone	hexahydrate	common motif<ref name=asia/>
MgSO4(H2O)7	[Mg(H2O)6]	one	epsomite	common motif<ref name=asia/>
TiOSO4(H2O)	[Ti(μ-O)2(H2O)(κ1-SO4)3]	Template:CNone		further hydration gives gels
VSO4(H2O)6	[V(H2O)6]	Template:CNone		Adopts the hexahydrite motif<ref>Template:Cite journal</ref>
VSO4(H2O)7	[V(H2O)6]	one		hexaaquo<ref>Template:Cite journal</ref>
VOSO4(H2O)5	[VO(H2O)4(κ1-SO4)4]	one
Cr(SO4)(H2O)3	[Cr(H2O)3(κ1-SO4)]	Template:CNone		resembles Cu(SO4)(H2O)3<ref>Template:Cite journal</ref>
Cr(SO4)(H2O)5	[Cr(H2O)4(κ1-SO4)2]	one		resembles Cu(SO4)(H2O)5<ref>Template:Cite journal</ref>
Cr2(SO4)3(H2O)18	[Cr(H2O)6]	six		One of several chromium(III) sulfates
MnSO4(H2O)	[Mn(μ-H2O)(μ4,-κ1-SO4)4]<ref name=Kieserite/>	Template:CNone	szmikite	see Fe, Co, Ni, Zn analogues
MnSO4(H2O)4	[Mn(μ-SO4)2(H2O)4]<ref>Template:Cite journal</ref>	Template:CNone	Ilesitepentahydrate is called jôkokuite; the hexahydrate, the most rare, is called chvaleticeite	with 8-membered ring Mn2(SO4)2 core
MnSO4(H2O)5		Template:Dunno	jôkokuite
MnSO4(H2O)6		Template:Dunno	Chvaleticeite
MnSO4(H2O)7	[Mn(H2O)6]	one	mallardite<ref name=Redhammer/>	see Mg analogue
FeSO4(H2O)	[Fe(μ-H2O)(μ4-κ1-SO4)4]<ref name=Kieserite/>	Template:CNone		see Mn, Co, Ni, Zn analogues
FeSO4(H2O)7	[Fe(H2O)6]	one	melanterite<ref name=Redhammer/>	see Mg analogue
FeSO4(H2O)4	[Fe(H2O)4(κ′,κ1-SO4)]2	Template:CNone		sulfate is bridging ligand, 8-membered Fe2O4S2 rings<ref name="ReferenceA"/>
FeII(FeIII)2(SO4)4(H2O)14	Template:Nowrap	Template:CNone		sulfates are terminal ligands on Fe(III)<ref>Template:Cite journal</ref>
CoSO4(H2O)	[Co(μ-H2O)(μ4-κ1-SO4)4]<ref name=Kieserite/>	Template:CNone		see Mn, Fe, Ni, Zn analogues
CoSO4(H2O)6	[Co(H2O)6]	Template:CNone	moorhouseite	see Mg analogue
CoSO4(H2O)7	[Co(H2O)6]	one	bieberite<ref name=Redhammer/>	see Fe, Mg analogues
NiSO4(H2O)	[Ni(μ-H2O)(μ4-κ1-SO4)4]<ref name=Kieserite/>	Template:CNone		see Mn, Fe, Co, Zn analogues
NiSO4(H2O)6	[Ni(H2O)6]	Template:CNone	retgersite	One of several nickel sulfate hydrates<ref>Template:Cite journal</ref>
NiSO4(H2O)7	[Ni(H2O)6]		morenosite<ref name=Redhammer/>
(NH4)2[Pt2(SO4)4(H2O)2]	[Pt2(SO4)4(H2O)2]2−	Template:CNone		Pt-Pt bonded Chinese lantern structure<ref>Template:Cite journal</ref>
CuSO4(H2O)5	[Cu(H2O)4(κ1-SO4)2]	one	chalcantite	main}}</ref>
CuSO4(H2O)7	[Cu(H2O)6]	one	boothite<ref name=Redhammer/>
ZnSO4(H2O)	[Zn(μ-H2O)(μ4-κ1-SO4)4]<ref name=Kieserite/>	Template:CNone		see Mn, Fe, Co, Ni analogues
ZnSO4(H2O)4	[Zn(H2O)4(κ′,κ1-SO4)]2	Template:CNone		sulfate is bridging ligand, 8-membered Zn2O4S2 rings<ref name="ReferenceA"/><ref>Template:Cite journal</ref>
ZnSO4(H2O)6	[Zn(H2O)6]	Template:CNone		see Mg analogue<ref>Template:Cite journal</ref>
ZnSO4(H2O)7	[Zn(H2O)6]	one	goslarite<ref name=Redhammer>Template:Cite journal</ref>	see Mg analogue
CdSO4(H2O)	[Cd(μ-H2O)2(κ1-SO4)4]	Template:CNone		bridging water ligand<ref>Template:Cite journal</ref>

Hydrates of metal nitrates

Transition metal nitrates form a variety of hydrates. The nitrate anion often binds to the metal, especially for those salts with fewer than six aquo ligands. Nitrates are uncommon in nature, so few minerals are represented here. Hydrated ferrous nitrate has not been characterized crystallographically.

Formula of hydrated metal ion nitrate	Coordination sphere of the metal ion	Equivalents of water of crystallization that are not bound to M	Remarks
Cr(NO3)3(H2O)9	[Cr(H2O)6]3+	three	octahedral configuration<ref>Template:Cite journal</ref> isostructural with Fe(NO3)3(H2O)9
Mn(NO3)2(H2O)4	cis-[Mn(H2O)4(κ1-ONO2)2]	Template:CNone	octahedral configuration
Mn(NO3)2(H2O)	[Mn(H2O)(μ-ONO2)5]	Template:CNone	octahedral configuration
Mn(NO3)2(H2O)6	[Mn(H2O)6]	Template:CNone	octahedral configuration<ref>Template:Cite journal</ref>
Fe(NO3)3(H2O)9	[Fe(H2O)6]3+	three	octahedral configuration<ref>Template:Cite journal</ref> isostructural with Cr(NO3)3(H2O)9
Fe(NO3)3)(H2O)4	[Fe(H2O)3(κ2-O2NO)2]+	one	pentagonal bipyramid<ref name=schm2012>Template:Cite journal</ref>
Fe(NO3)3(H2O)5	[Fe(H2O)5(κ1-ONO2)]2+	Template:CNone	octahedral configuration<ref name=schm2012/>
Fe(NO3)3(H2O)6	[Fe(H2O)6]3+	Template:CNone	octahedral configuration<ref name=schm2012/>
Co(NO3)2(H2O)2	[Co(H2O)2(κ1-ONO2)2]	Template:CNone	octahedral configuration
Co(NO3)2(H2O)4	[Co(H2O)4(κ1-ONO2)2	Template:CNone	octahedral configuration
Co(NO3)2(H2O)6	[Co(H2O)6]2+	Template:CNone	octahedral configuration.<ref>Template:Cite journal</ref>
α-Ni(NO3)2(H2O)4	cis-[Ni(H2O)4(κ1-ONO2)2]	Template:CNone	octahedral configuration.<ref>Template:Cite journal</ref>
β-Ni(NO3)2(H2O)4	trans-[Ni(H2O)4(κ1-ONO2)2]	Template:CNone	octahedral configuration.<ref>Template:Cite journal</ref>
Pd(NO3)2(H2O)2	Template:Nowrap	Template:CNone	square planar coordination geometry<ref>Template:Cite journal</ref>
Cu(NO3)2(H2O)	[Cu(H2O)(κ2-ONO2)2]	Template:CNone	octahedral configuration.
Cu(NO3)2(H2O)1.5	Template:Dunno	Template:Dunno	Template:Dunno<ref>Template:Cite journal</ref>
Cu(NO3)2(H2O)2.5	[Cu(H2O)2(κ1-ONO2)2]	one	square planar<ref>Template:Cite journal</ref>
Cu(NO3)2(H2O)3	Template:Dunno	Template:Dunno	Template:Dunno<ref>J. Garaj, Sbornik Prac. Chem.-Technol. Fak. Svst., Cskosl. 1966, pp. 35–39.</ref>
Cu(NO3)2(H2O)6	[Cu(H2O)6]2+	Template:CNone	octahedral configuration<ref>Template:Cite journal</ref>
Zn(NO3)2(H2O)4	cis-[Zn(H2O)4(κ1-ONO2)2]	Template:CNone	octahedral configuration.
Template:Chem2	[H2O–Hg–Hg–OH2]2+	linear<ref>Template:Cite journal</ref>

Photos

• Copper sulfate.jpg

  Hydrated copper(II) sulfate is bright blue.

• Copper sulfate anhydrous.jpg

  Anhydrous copper(II) sulfate has a light turquoise tint.

See also

• Hydrate
• Mineral hydration
• Hydrous oxide